Image forming apparatuses as represented by printers, facsimile machines, and copiers are used for forming images including characters and symbols on recording media such as sheets, fabrics, and OHP sheets based on image information. Particularly, electrophotographic image forming apparatuses have become widespread at offices because they can form high-definition images on plain sheets. Such electrophotographic image forming apparatuses generally use a heat fixing method in which toner on a recording medium is heated and melted and the melted toner is then pressurized to fix the toner to the recording medium. This heat fixing method has been preferably used because it can provide high fixing speed, high fixing image quality, etc.
However, approximately more than half of power consumption in such electrophotographic image forming apparatuses is caused when toner is heated in the heat fixing method. On the other hand, from the viewpoint of solving environmental problems in recent years, low power consumption (energy-saving) fixing units have been demanded. In other words, a fixing method, which greatly lowers temperature for heating toner to fix toner more than ever before or which does not require heating the toner at all, has been demanded. Particularly, it is ideal to provide a non-heating fixing method for fixing toner to a recording medium without heating the toner at all from the viewpoint of low power consumption.
As such a non-heating fixing method, Patent Document 1 has proposed a wet fixing method. Specifically, in this wet fixing method, an oil-in-water type fixing agent, which is capable of dissolving or swelling toner and in which a water-soluble or poor water-soluble organic compound is dispersed and mixed in water, is sprayed or dropped from the front surface of an object to be fixed on which unfixed toner particles are placed at a given position so as to dissolve or swell the toner, and then the object to be fixed is dried.
However, the wet fixing method of Patent Document 1 uses the oil-in-water type fixing agent in which the water-soluble or poor water-soluble organic compound is dispersed and mixed in water. Therefore, when a large amount of fixing agent is applied to the unfixed toner particles, a recording medium (object to be fixed) such as a transfer sheet absorbs water of the fixing agents, thereby being wrinkled or curled. As a result, stable and high-speed conveyance of the recording medium needed for an image forming apparatus is remarkably degraded. To this end, when the large amount of water included in the fixing agents is evaporated with a drying unit to remove the water of the fixing agents applied to the recording medium, the power equivalent to the power consumption of the image forming apparatus using the heat fixing method is needed. Furthermore, as water-repellent fixing liquids that reject unfixed toner particles, some oily fixing liquids have been proposed in which a material that causes toner to be dissolved or swelled is dissolved in an oily solvent. By way of example, Patent Document 2 has proposed a fixing liquid in which an aliphatic dibasic acid ester, which has a material that causes a resin component constituting toner to be dissolved or swelled as an component, is diluted (dissolved) with non-volatile dimethyl silicon serving as a dilution liquid (solvent). Furthermore, as a fixing liquid used for a fixing method in which an unfixed image formed with an electrostatic method can be clearly and easily fixed to an image receiving sheet without causing irregularities in an image, Patent Document 3 has proposed a fixing liquid for a compatible unfixed toner image in which 8 through 120 parts by volume of silicon oil are mixed in 100 parts by volume of a solvent that dissolves toner and has compatibility with the silicon oil. Such an oily fixing liquid includes an oily solvent having a high affinity for water-repellent toner. Therefore, the oily fixing liquid can cause toner to be dissolved or swelled without rejecting the water-repellent toner and fix the toner to a recording medium.
Patent Documents 1 through 3 disclose inventions in which a fixing liquid is configured to be applied to an toner layer. Here, as shown in FIGS. 1A and 1B, the fixing liquid is applied to the toner layer 83 on the recording medium 82 with a coating roller 81 serving as a contact application unit. When a fixing liquid layer 84 on the coating roller 81 is made thinner than the toner layer 83 for applying a small amount of fixing liquid to the recording medium 82, unfixed toner particles are pulled up by the surface tension due to the liquid film of the fixing liquid on the front surface of the coating roller 81 at the position where the coating roller 81 is separated from the recording medium 82. Therefore, the toner particles are offset on the front surface of the coating roller 81, which causes much irregularities in an image on the recording medium.
Conversely, when the fixing liquid layer 84 on the coating roller 81 is made sufficiently thicker than the toner layer 83, the surface tension due to the liquid film on the front surface of the coating roller 81 is not likely to directly act on the toner particles at the position where the coating roller 81 is separated from the recording medium 82, thereby preventing the offset of the toner particles on the side of the coating roller 81. However, in this case, a large amount of fixing liquid is applied to the surface of the recording medium 82, which results in the toner particles flowing over the recording medium 82 due to the excessive fixing liquid which causes degradation in image quality or takes a long time for drying the fixing liquid which causes a problem in fixing response. In addition, a remarkable residual liquid feeling (wet feeling at the time of touching the recording medium 82 by hand) is caused on the recording medium 82. Moreover, when a large amount of fixing liquid containing water is applied to the recording medium such as a sheet containing cellulose, the recording medium is remarkably curled, which may cause jamming when the medium is conveyed, for example, in an image forming apparatus. Accordingly, when the fixing liquid is applied to the recording medium with the coating roller 81, it is really hard not only to apply a small amount of fixing liquid to the toner layer on the recording medium but also prevent the offset of the toner particles to the fixing roller so as to improve the fixing response, reduce the residual liquid feeling, and prevent the recording medium from being curled. Even if a die-coat unit, a blade coating unit, or a wire bar coating unit is used as the contact application unit, toner particles are caused to adhere onto the fixing roller due to the surface tension when a small amount of fixing liquid is applied, thereby causing degradation in image quality.
As described above, in the conventional method for applying the fixing liquid, it is really hard not only to apply the small amount of fixing liquid to the toner layer on the recording medium so as to improve the fixing response but also to uniformly apply the fixing liquid without causing the irregularities in a toner image with the contact application unit. This problem is not limited to the toner on the recording medium, but it could occur in any case where an aquiform fixing liquid is applied to the layer of resin-containing fine particles on a recording medium.
According to Patent Document 4 having been proposed to solve these problems, a foamed fixing liquid is applied to resin-containing fine particles of toner, etc., on a medium such as a sheet. Accordingly, after the fixing liquid is applied to the medium on which the resin-containing fine particles adhere, the resin-containing fine particles are quickly fixed to the medium.    Patent Document 1: JP-B2-3290513    Patent Document 2: JP-A-2004-109749    Patent Document 3: JP-A-59-119364    Patent Document 4: JP-A-2007-219105    Non-Patent Document 1: P.489, first edition of “Concepts in Basic Bubble and Foam Engineering” written by Toshio Ishi and published on Mar. 25, 2005, by Techno system Corporation
However, a softening agent contained in the fixing liquid has a strong defoaming effect. Therefore, as the concentration of the softening agent in the fixing liquid increases, the foaming property and the foaming stability of the fixing liquid are degraded. As a result, foams are hardly generated. In addition, a foamed fixing liquid having low foam density cannot be obtained because foams easily disappear.